This invention relates to a sensor for use in the measurement of the radio frequency (RF) current flowing in a conductor, for example in plasma processes such as those used to fabricate semiconductor devices.
Plasmas are widely used to process materials, including etching and depositing layers. Many of the plasma processes are critical steps in the production of electronic devices.
Production plasmas are normally produced by the application of RF voltages (0.1 to 100 mHz) to low pressure gas reactors. The electrical or electro-magnetic field causes a breakdown in the gas and aids the surface modification, etching or deposition of the material in a controlled way. As well as producing the plasma, high frequency voltages are also used to bias the surface being processed in order to enhance the required process. It is well known in the prior art that the current and voltage at the process surface are correlated with process rates, process quality, and surface damage. The current and phase in the absence of a plasma are determined by the chamber impedance and applied voltage. The chamber impedance is also a critical parameter in determining process quality or fault detection/classification.
The plasma is a conducting gas containing electrons and ions that are free to conduct electrical current. The ions have lower mobility than the electrons and the plasma has a current-voltage characteristic similar to a diode. This diode characteristic tends to rectify the applied radio frequency current and generate harmonics. The intensity of the harmonics is correlated to plasma parameters and other process parameters of interest to the process engineer. The harmonics also have an impact on the accuracy of conventional sensors and can cause unwanted damage to circuitry.
In general, in industry the current state of the art recognises the importance of accurate control of chamber impedance, current and voltage levels and harmonic content (see, for example, U.S. Pat. Nos. 5,314,603, 5,472,561, 5,866,985 and 5,910,011). However, the lack of accurate, robust and cost effective sensing and measuring circuits at high power with non-standard and non-linear load impedance have limited the ability of industry to apply many of these techniques. This limits the repeatability of plasma process and has a yield impact that cost industries such as the Semiconductor industry many millions of dollars annually.
There is a need in many industries using plasma processes generated by radio frequency (RF) voltages to measure accurately the current, voltage and the phase between them, at the applied frequency and at higher harmonics in varying non-linear reactive and inductive loads. In some cases several variable frequencies are applied simultaneously.
In principle, voltage and current measurements are straightforward. The standard approach, as described in U.S. Pat. No. 5,325,019, uses a single loop for current sensing and a capacitor for voltage sensing.
An improvement to this circuit is described in U.S. Pat. No. 5,808,415 and uses a sensor having two pick-up loops to increase the accuracy of the current detection. In this case the current-carrying conductor is divided in the region of the sensor and the sensor is inserted between the two parts of the conductor so that the loops lie one on each side of the conductor. The sensor carries the two loops as a pattern of conductive material formed on a layer of insulating material. This allows for a cheap robust sensor that has substantial reduced pick-up of stray field due to external sources of RF or through non-symmetries in the RF field surrounding the current-carrying conductor.
A need now exist for a sensor capable of more accurate measurement of RF current, for use, for example, in the control of increasingly complex plasma processes.
Accordingly, the present invention provides a sensor for detecting RF current flowing in a conductor, the sensor comprising at least one layer of insulating material, a hole through the layer(s) for the passage therethrough of the RF conductor in a direction substantially normal to the layer(s), a plurality of conductive elements on at least two major surfaces of the layer(s), and a plurality of through-holes in the layer(s) each containing conductive material, the through-holes being so positioned that the conductive material selectively connects the conductive elements on said at least two major surfaces to form with said elements a plurality of loops disposed non-parallel to the layer(s) for inductive coupling to the RF conductor.
Preferably the loops are connected in series and, most preferably, substantially completely surround the hole.
Preferably at least three and most preferably at least eight loops are formed, but the invention allows the fabrication of an arbitrary number of loops and, in general, the greater the number of loops the greater the immunity to external fields or non-uniformity in the magnetic field around the current-carrying conductor. Further, this is achieved without the need to split the current-carrying conductor.
In an embodiment the sensor also incorporates an RF voltage sensor in the form of a layer of conductive material on a surface of one of the layers for capacitive coupling to the RF conductor.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: